The most typical example of a device that produces a counter or back electro-motive force (back emf) is a d.c. motor. The invention is described in terms of a d.c. motor, but is to be understood that this example is not to be construed as limiting the devices capable of producing a back emf.
A familiar apparatus for supplying power to a d.c. motor, to control forward and reverse running of the motor, includes two d.c. power supply sources and two opposite conductivity type power transistors. The power transistors and power sources are connected in series in a loop so the motor is connected between a common terminal for the sources and a common terminal for like electrodes of the transistors. The transistors are activated so that one transistor is conducting while the other transistor is cut off so the motor is supplied with current from one of the d.c. sources to cause the motor to run in a first direction; the motor runs in a second, reversed direction in response to the opposite condition subsisting for the transistors, whereby current flows in the opposite direction through the motor from the second d.c. source. Generally, this type of circuitry includes a control circuit which activates, by pulses, one of the transistors to the exclusion of the other. In addition, to establish a reference potential, typically at ground level, the common terminal for like electrodes of the transistors is grounded.
Prior art devices of this type have numerous disadvantages. In particular, the series connection of the two power transistors limit the power which can be supplied to the motor because the maximum voltage which can be applied to the motor is equal to the sum of the supply voltages of the two d.c. power sources that are connected to the transistors and the motor. Another restriction is imposed by the maximum current which the two power transistors are able to supply when the back emf is at a maximum value, corresponding with a maximum speed for the motor. Because of the maximum voltage and maximum current restrictions, if the back emf rises to a maximum value simultaneously with a large torque being exerted on the motor shaft, the energy which can be supplied by the power transistor to the motor, while the motor is accelerating, is relatively low. Depending upon the torque magnitude, the energy supplied by the power transistor to the motor may be so small that a feedback loop responsive to the motor speed inaccurately represents the motor speed. Also, the relatively small margin within which transistors are able to operate while the motor is generating a large back emf imposes a corresponding restriction on the motor operation, both with regard to shaft speed and torque that the shaft can apply to a load. If the transistors are activated so that the motor is decelerated from a running condition, the back emf is added to the power supply voltage. Because the back emf and power supply voltage are added together, there are enormous peak currents when it is only desired to reduce the motor speed slightly. Hence, there is a major disparity between the actual and desired operating characteristics of a motor when it is supplied by the prior art device. In particular, it is not possible to overcome the high resisting torque with the prior art arrangement, nor is it possible to have a wide range of speeds.